Thank you. In a sudden burst of non-stupidity, I decided to use the variable that the game already uses to keep track of the player's current powerup, instead of some ill-informed choice of memory (Durrr? What does ROM stand for again?). That fixed it! It now works exactly as intended.

I'll probably have more questions as I attempt more ambitious things down the road, but thanks to everyone for chiming in.


By the way, can someone recommend a good assembler for NES?